Originally posted by Corse-R
You can go down up to .021" with a 30%, but plug selection will be critical (you're on the ragged edge for 30% Nitro). Better to go with a Nova 7Tf with those nitro percentages. You can go down up to .018, but will be quite risky and will be quite weather sensitive.
Corse,
I usually run my NR based engines with 0,3 mm shims, 30% nitro and the shortbody 5TF plug without the slightest problems.
That is in Georgia weather where the temperatures reach 100F with 95% humidity.
Never had detonations or premature wear on my engines.